If this is on the right hand side check to make sure you don't have a hole on the clean grain elevator. Had the same thing happen on my 7720 up on the top where it dumps into the auger.
855man my first thought would be over loading but your figures would say other wise..if you are happy with your sieve opening i would inspect the rubber flaps on the front of back screen you may find a little friend has had a bit of a chew allowing grain to spill forward we had niegbours 7720 with this problem good luck